1. Field of the Invention
The present invention relates to a sheet feeding apparatus mounted on an image reading apparatus such as a copying machine and a scanning apparatus, or an image forming apparatus such as a printer and a facsimile apparatus, for sequentially taking in a sheet such as a document contained in a stacked state in sheet containing means and supplying and conveying it to a conveyance path. The invention also relates to an image reading apparatus and an image forming apparatus provided with the sheet feeding apparatus.
2. Description of Related Art
Recently, an apparatus which automatically takes in a sheet of document one by one, conveys it and sequentially reads an image of the document is mounted on an image forming apparatus or the like in order to efficiently read the image or to form an image based on the read image. In the meantime, as digital technology has been developed, the speed of image reading from the document, of conversion from the read data to electronic data and of image formation from the electronic data is increased. According to such a reading apparatus, in order to perform processing for the larger number of documents at high speed, the number of documents which can be set once is increased to as many as 100 to 200 sheets. Then, even when the number of sheets loaded is increased, in order to uniformly provide a taking-out condition by sheet supplying and conveying means, sheet containing means is enabled to be elevated or lowered by elevation/lowering driving means and controlling means controls the elevation/lowering driving means to maintain an upper surface height of the sheet at a predetermined height (referring to Japanese Patent Application Laid-Open Nos. 11-237771 (1999), 10-250853 (1998) and 7-17640 (1995), for example). In addition, as a document conveying unit in a document reading apparatus has been developed, kinds of the documents which can be conveyed are diversified.
However, according to the above apparatus which can set many sheets, when the sheet containing means is put on standby at the lowest position, capacity of the sheets can be increased, but it takes time to elevate the sheet conveying means in order to put the sheet containing means into a state in which the document can be supplied to a conveyance path, after a start signal for reading was received through a key operation or the like. Consequently, there is a problem that a total reading time for reading the document is increased.
Thus, in order to solve such a problem, according to Japanese Patent Application Laid-Open No. 11-237771, for example, a tray is automatically elevated when a predetermined time passed after it was detected that the document was set in the tray serving as the sheet containing means, and the tray is controlled so that elevation is completed before a document reading signal is outputted, to decrease the total reading time. However, according to the prior art, when it becomes necessary to check the document, add or reduce the document, correct a set state of the document (when it is not set straight or the like) or the like after the document was set, since the document is in a state pressed by a guide-in roller, there is a problem that the above operations cannot be easily performed.
Normally, in such a case, the user pulls out the document by force. At this time, however, the document could be damaged or contaminated or the guide-in roller could be damaged, which is not preferable. In this respect, it can be considered that a key for lowering the tray is provided in the operation panel or the like, but there is a problem that the user is not likely to notice the key.
Then, as another prior art, according to Japanese Patent Application Laid-Open No. 10-250853, an upper surface height of the document is detected and the document is put on standby at the height in the vicinity of the higher limit in which the document is not in contact with a guide-in roller. However, according to this prior art, although the document can be pulled out or aligned, there is a problem that it is sometimes difficult to add the document and a sensor is necessary.